The adhesive layers is used to draw where glue should be dispensed under component body in case the component needs to be fixed to the board before soldering. This may be the case for larger SMD components on the bottom side of an SMD board that is populated on both sides or for wave soldered SMD boards.
The main problem with the adhesive graphics is that all aspects and details of gluing is highly process dependent. It is very unlikely the PCB designer or the footprint designer could figure when to use the glue and what the right amount of glue is or even how the glue is dispensed (using stencil or nozzle).
Thus drawing the adhesive layer is optional. When drawn for a general purpose footprint, it should be done with either as a thin line which indicates the centerline or multiple small dots (zero length lines of reasonably small but visible diameter). The intention is that the fab house could import this layer as a hint on where gluable components are on the board and use relatively simple transformations (increasing line thickness) to tune glue for their actual process.